Transistor gate electrodes made of doped polysilicon have long been used in the manufacture of metal oxide semiconductor (MOS) transistors. The use of doped polysilicon gates becomes problematic, however, as the dimensions of gates and gate dielectrics are reduced. In particular, small polysilicon gates can accommodate only a finite amount of dopant material. This limitation can in turn result in a depletion of gate charge carriers at the interface between the gate and gate insulator (dielectric), when the gate electrode of a device is biased to invert the channel. Consequently, the electrical thickness of the gate dielectric is substantially increased, thereby deteriorating the performance characteristics of the transistor, such as reducing the drive current and switching speed. Depletion of the polysilicon gate is thus a fundamental issue that limits further scaling of MOS devices.
Metal gate stacks are an attractive alternative to polysilicon gates since they have a larger supply of charge carriers than doped polysilicon gates. One existing type of metal gate stack structure includes both a lower metal layer and an upper polysilicon layer. When a metal gate stack is inverted, there is no substantial depletion of carriers at the interface between the metal gate layer and the gate dielectric. Accordingly, the transistor's performance is not deteriorated because the electrical thickness of the gate stack is not increased. However, the fabrication of such metal gate stack structures presents its own set of unique challenges due to the nature of the components. For example, for “gate first” integration schemes, a thin metal layer (e.g., TiN) is deposited over a gate insulating layer and substrate, followed by a thicker layer of polysilicon. The thin metal layer, as well as the insulating layer, typically need to be undercut in order to prepare for the formation of the spacers along the sides of the gate stack. Conventional techniques for undercutting these layers are time consuming.
It is within this context that the invention arises.